The present invention relates to the field of adjustable imaging lenses.
It is desired to provide a single multi-element lens device which can readily correct its own chromatic aberration over any of the astronomical U, B, V, and R wavelength bands. Such a lens is intended to be used with a set of astronomical filters, and is adjustable in a manner similar to a zoom lens. However, the motion of the lens elements changes the center wavelength at which the imagery is well-corrected rather than the size of the image. This correction is accomplished by a change in spacing between the second and third element of the lens. The spacing is changed as the optical bandpass filters are exchanged for one another.
A camera with a charge coupled device or CCD was to be used in a large telescope and it was desired to use the full wavelength range of the CCD, nominally 0.4 to 0.9 micrometers. A reflective system, insensitive to wavelength, was not practical due to space limitations, and no existing refractive lens system could cover the wavelength range with good imagery. Because the CCD camera was used in astronomical imaging, a U, B, V, or R filter would be used to limit the bandpass of the light into the system. Thus, an appropriate lens only had to correct chromatic aberration well, over a set of limited bands, rather than the entire useful bandpass of the CCD.
In the course of performing the optical design of a system, it is usual to vary the spacing between two lens elements of the lens system to achieve the best chromatic performance over a fixed wavelength bandpass. The best spacing is then fixed. A distinguishing feature of the lens device of the present invention is that the spacing of two lens elements is mechanically varied to accommodate different wavelength bandpasses.